A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements for transmitting an ultrasound beam and receiving a reflected beam from the object being studied. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector location and direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound energy. The voltages produced at the receiving elements are summed so that the net signal is indicative of the ultrasound reflected from points in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving element. The receive delays may be modified during reception to dynamically increase the focal depth as echoes are received from progressively deeper points along a line within the transmit beam.
Recently, many conventional ultrasound imaging systems have included a two-dimensional transducer array (hereinafter a 2D transducer array). The 2D transducer array typically comprises a number of transducer elements arranged in a grid. By controlling the timing and amplitude of the elements in the 2D transducer array, it is possible to steer and translate the transmitted ultrasound beam in both an azimuth direction and in an elevation direction. The use of a 2D transducer array allows the ultrasound transducer or probe to have greater flexibility and it enables greater accuracy in the acquisition of volumetric data.
However, for some ultrasound systems and probes, the number of transducer elements exceeds the number of channels in the console beamformer electronics or the number of channels supported by the console interface. For example, a 2D transducer array used for 3D and 4D imaging may require a very high number of elements, roughly the square of the number of elements needed for a 1D array used for 2D imaging. For example, a linear array which would required 128 to 192 elements for 2D imaging would need approximately 8,000 to 10,000 elements for 3D and 4D imaging. In cases like this, one or more probe beamforming and/or switching circuits may be used to dynamically couple the available channels to different subsets of transducer elements during different portions of the image formation process. Even if a probe beamforming circuit, also known as a sub-aperture processor (SAP), is used to combine 10 or more elements for each console beamformer channel, there may still not be enough console beamformer channels to utilize all of the elements in the 20 transducer array.
Additionally, while scanning a single slice, such as for a 2D display, it is often desirable to optimize the resolution within the slice. One way to optimize the resolution is to use a receive aperture that has its widest extent in the scanning dimension. For example, when scanning in the azimuth direction, it may be desirable to have a receive aperture that is widest in the azimuth direction. Likewise, when scanning in the elevation direction, it may be desirable to have a receive aperture that is widest in the elevation direction. Additionally, when scanning a volume to render as a 3D or 4D image, it may be desirable to optimize the resolution for more uniformity in both scanning dimensions by using an aperture that is shaped more like a square. For these and other reason, there is a need for an easily configurable ultrasound imaging system with the flexibility to optimize the shape of the receive aperture of a 2D transducer array based on the type of image that is desired.